Paper companies are continually seeking to improve the brightness and whiteness of their paper grades, especially printing and communication papers. The most common way of improving brightness at present is by increasing the amount of optical brightening agents (OBA's) or fluorescent brightener/whitener agents (FWA's) either at the wet end or at the size press. In many cases, this requires adding significantly high amounts of OBA's. However, there are drawbacks to adding large amounts of OBA's, such as the effect on the white water (recycle water) and changes to the paper making system charges. Also, the cost and availability of OBA's is a concern, since OBA's are not only expensive, but in great demand and supply is limited.
Paper mills tend to follow a general procedure rather than a customized procedure for chemical addition, often resulting in the mills using too much OBA as their main means of improving the brightness and whiteness of the paper. Moreover, in order to compete with new paper grades having increased brightness and/or whiteness, paper mills generally believe that the only way to improve brightness and whiteness is to keep increasing the OBA levels. Therefore, there is a need to find alternative ways of increasing the brightness and whiteness, without increasing, and preferably even reducing, the amount of OBA being used.
The paper making process involves many variables that can affect the optical quality of the final paper. The selection of the species of the tree(s) will have a tremendous impact on the final paper grade, including the ultimate brightness and whiteness. It is well known that increased pulp refining operations causes brightness loss in the pulp. However, refining is needed among other things to increase paper strength, fiber to fiber bond, increase smoothness, and improve formation. Fine paper mills refine to a greater degree to obtain properties such as opacity, porosity and strength. Some mills have to refine to a certain freeness to meet key operating parameters and have very little room for change. Pulp brightness also affects the final paper brightness, i.e., the brighter the pulp the brighter the paper. Therefore, losing pulp brightness due to refining has a serious impact on the final paper brightness.
In EP 1 378 545 A1, a specific aqueous liquid composition of a hexasulfonate fluorescent brightening agent is disclosed. It is further disclosed that the liquid composition can be added either internally to the pulp or, alternatively, externally in the size press coating. However, there is no teaching or suggestion of how to obtain high brightness and whiteness for highly refined pulp or of using other optical brightening agents or specific ratios or sequences of either wet end or size press additives to achieve high brightness or whiteness with such highly refined pulp.
EP 1 086 825 A1 is directed to an ink jet recording paper which is coated with a coating solution containing a fluorescent brightening agent, a water-soluble binder and a cationic polymer fixing agent. However, there is no teaching or suggestion of how to obtain high brightness and whiteness for highly refined pulp or of using specific ratios or sequences of either wet end or size press additives to achieve high brightness or whiteness with such highly refined pulp.
Despite considerable efforts which have been applied with the available products to solve the problem, there still exists a need to preserve brightness and whiteness during refining and to increase the brightness and whiteness of paper in a most efficient manner without increasing the OBA usage level.